1. Field of the Invention
The present invention relates to a driving circuit of an ultrasonic motor, for driving an ultrasonic motor.
2. Description of the Related Art
There has conventionally been known an ultrasonic motor which employs ultrasonic vibration as driving force. In a traveling-wave ultrasonic motor which is a kind of ultrasonic motor, a stator is formed with a piezoelectric body attached to a ring-shaped elastic body and a rotor mounted on a driving shaft press-contacts the stator.
The driving circuit of the ultrasonic motor supplies, to the piezoelectric body, driving signals of two phases (sine wave and cosine wave) which differ from each other by 90 degrees at a predetermined frequency. Due to mechanical vibration of the piezoelectric body caused by the driving signals of two phases, the elastic body is excited by ultrasonic vibration (traveling wave) in which the antinode and node of the vibration move annularly along the elastic body. The traveling wave causes the rotor, which press-contacts the elastic body, and the driving shaft to rotate.
It may be thought that such an ultrasonic motor might be used in, for example, a so-called tilt mechanism of a steering device of an automobile, a telescopic mechanism, and the like. One example of a driving circuit of such an ultrasonic motor is shown in FIG. 3.
As shown in FIG. 3, a driving circuit 30 is comprised of a microcomputer 32, an oscillator circuit 34, a switching control circuit 36, a band pass filter 40, an A-phase amplifier circuit 42, a B-phase amplifier circuit 44, and a voltage control circuit 38.
The microcomputer 32 outputs a driving frequency signal to the oscillator circuit 34 and the oscillator circuit 34 oscillates at a driving frequency designated by the microcomputer 32. The switching control circuit outputs switching signals to the A-phase amplifier circuit 42 and also to the B-phase amplifier circuit 44 at a predetermined timing. The A-phase amplifier circuit 42 and the B-phase amplifier circuit 44 each convert a DC (direct current) voltage supplied by the voltage control circuit 38 to an AC (alternating current) voltage and supply the AC voltage to each of piezoelectric bodies 14A and 14B of an ultrasonic motor 10.
When the above-described AC voltage is supplied to each of the piezoelectric bodies 14A and 14B of the ultrasonic motor 10, the ultrasonic motor 10 is driven.
A small aperture is formed in a region where a driving shaft and a rotor contact each other. For this reason, there is a drawback in that, when the ultrasonic motor is rotated at high speed at the time of the start of driving thereof, an audible sound may be generated due to the driving shaft and the rotor contacting each other.
In order to solve the above-described drawback, there is a method in which, as shown in FIG. 7, at the time of the start of driving, first, a driving signal whose frequency is sufficiently higher than an audible sound generation band is supplied to the piezoelectric body 14 to rotate the motor at a low number of revolutions (see FIG. 8), and a driving frequency is gradually lowered into a driving frequency band which is slightly higher than the audible sound generation band so as to gradually increase the number of revolutions, and further, the driving frequency is controlled so as to be maintained within the driving frequency band, thereby preventing generation of audible sound.
However, in the above-described method, it is not possible to completely remove the audible sound which is generated at the time of the start of driving the ultrasonic motor 10. The reason is that, at the time of the start of driving the ultrasonic motor 10, the rotor and the stator which press-contact each other are separated from each other, and the vibration generated at this time causes audible sound.
Moreover, at the time of the start of driving the ultrasonic motor 10, there exist such drawbacks as described below.
First, there is shown in FIG. 21 an example of each variation of a rotational pulse signal corresponding to the rotation of the ultrasonic motor 10, which is outputted from a rotation sensor 46 at the time of the start of driving the ultrasonic motor 10 in a conventional method, a speed indicating value (a driving frequency signal) outputted by the microcomputer 32 which controls the ultrasonic motor 10, and of a rotational speed of the ultrasonic motor 10. As shown in FIG. 21, when the speed indicating value outputted by the microcomputer 32 gradually increases, the rotational speed of the ultrasonic motor 10 also gradually increases. At the same time, the microcomputer 32 calculates the rotational speed based on the rotational pulse signal outputted from the rotation sensor 46. Meanwhile, the wider the pulse width of the rotational pulse signal, the slower the rotational speed becomes, and the narrower the pulse width, the higher the rotational speed becomes. The speed indicating value is increased until the rotational speed comes to a predetermined rotational speed.
On the other hand, there is a small difference between the timing at which the rotational speed of the ultrasonic motor 10 comes to a predetermined rotational speed and the timing at which it is determined by the microcomputer 32 that the rotational speed of the ultrasonic motor 10 comes to a predetermined rotational speed. For this reason, when it is determined by the microcomputer 32 that the rotational speed of the ultrasonic motor 10 comes to a predetermined rotational speed, there is a possibility that an actual rotational speed of the ultrasonic motor has already exceeded the predetermined rotational speed. In this case, there exists a drawback in that the driving frequency is excessively lowered to become a frequency in the audible sound generation band, which results in the generation of an audible sound.
Further, at the time of stoppage of driving the ultrasonic motor 10, there exist such drawbacks as described below.
When the ultrasonic motor 10 is stopped, it suffices that a switching signal is turned off to cause a step-up voltage outputted from the voltage control circuit 38 to be set at 0 and a driving voltage for the ultrasonic motor 10, which is to be outputted from each of the A-phase amplifier circuit 42 and the B-phase amplifier circuit 44, is further set at 0. However, when the switching signal is turned off, the AC voltage supplied to each of the piezoelectric bodies 14A and 14B of the ultrasonic motor 10 suddenly becomes 0 and the vibration of the piezoelectric bodies 14A and 14B of the ultrasonic motor 10 suddenly stops. As a result, an audible sound is generated due to the stator and the rotor contacting each other.
Moreover, when a battery source for a vehicle is used, the battery voltage may become unstable, which affects adversely the step-up voltage outputted from the voltage control circuit 38 and the AC voltage outputted from each of the A-phase amplifier circuit 42 and the B-phase amplifier circuit 44. For this reason, it is difficult to stop the ultrasonic motor 10 smoothly.
In order to solve the above-described drawbacks, there can be considered a method for insulating the noise of an entire ultrasonic motor, and the like. However, such method has the disadvantage of an increase in cost.
Further, when the ultrasonic motor is applied to the tilt mechanism of a steering device, it may be supposed that the ultrasonic motor is stopped by locking a steering wheel at a lock position and by detecting a decrease in the number of revolutions of the ultrasonic motor. However, in this case, there are such drawbacks as described below.
First, an example of variations, with the passage of time, of a driving signal outputted by the microcomputer 32 and of the number of revolutions of the ultrasonic motor 10 in a conventional method is shown in FIG. 18. When a steering wheel is moved and locked at a lock position at t0, the number of revolutions of the ultrasonic motor 10 gradually decreases. At this time, the microcomputer 32 calculates the number of revolutions at intervals of a predetermined time (i.e., calculates at each timing of t1 to t5). When the number of revolutions comes to the above-described predetermined number of revolutions V.sub.stop or less at t5, the driving signal is turned off to stop driving the ultrasonic motor 10.
On the other hand, in order to prevent driving of the ultrasonic motor 10 from mistakenly stopping when the rotational speed of the ultrasonic motor 10 becomes a predetermined rotational speed or less due to other factors, a predetermined number of revolutions V.sub.stop is set at a rather low value. For this reason, the period of time T from the time of locking the steering wheel at the lock position to the time of stoppage of driving the ultrasonic motor 10 becomes longer, thereby resulting in the generation of an audible sound.
In order to solve the above-described drawback, there may be considered a method in which a sensor is provided at the position where the steering wheel is locked and driving of the ultrasonic motor 10 is stopped by a detection signal of the sensor. However, in this case, there further arises a drawback of an increase in cost.
Moreover, in addition to the above-described drawback in that the audible sound is generated at the starting time of driving and at the stopping time of driving, there also exist the following drawbacks.
When an abnormal sound is generated from the ultrasonic motor 10, an audible sound signal is included in a feed-back signal outputted from a piezoelectric element 26 and is very feeble as compared with a driving frequency signal of the ultrasonic motor 10. It becomes necessary that the feeble audible sound signal is detected to control the number of revolutions of the ultrasonic motor 10 and generation of the abnormal sound from the ultrasonic motor 10 is prevented.
On the other hand, for example, when a wire for connecting the ultrasonic motor 10 and the driving circuit 30 of the ultrasonic motor 10 becomes longer, there exists a drawback in that electrostatic induction occurs so that an amplitude level of the driving frequency signal of the ultrasonic motor 10 becomes larger, and detection of the feeble audible sound signal is not possible.
Further, when the audible sound signal is detected from the feed-back signal, the speed indicating value is decreased and the driving frequency is increased, so that the driving frequency is separated from the audible sound generation band. However, when the ultrasonic motor 10 is driven at an unchanged speed indicating value, the rotational speed of the ultrasonic motor 10 remains low. For this reason, in the case in which the audible sound signal is detected from the feed-back signal, when the speed indicating value is instantly increased and the driving frequency is lowered, the driving frequency immediately approaches the audible sound generation band, thereby resulting in the possibility that the audible sound may be generated again.